Optical lithography is one of the technologies that are being used in the production of microchips. It uses a ‘reticle’, also called ‘photomask’ or ‘mask’ to form certain patterns in a ‘photo-resist layer’ that is coated on a ‘wafer’. This mask contains a pattern that, when an image of it is projected on the wafer, generates the desired pattern in this photoresist layer, after it has been chemically developed. This projected image is formed on the photoresist layer by illuminating the mask with a certain wavelength and light-source shape. The light passing through the mask is then captured by the projection lens of the lithography exposure tool, and this lens forms an image of the mask pattern in the photoresist layer. The mask itself consists of an optically-transparent plate on which patterns have been created at one side: these patterns consist of polygons in which the optical-transmission properties of the mask are modified with respect to the ‘unpatterned areas’. As an example, these polygon-shaped mask patterns might consist of a thin layer that absorbs or attenuated the light that passes through them.
The above basic concept for optical-projection-lithography is however complicated by two elements. The shapes in the image that is formed on the wafer, is never an identical copy of the shape of the patterns on the mask, and the difference between the two has become larger as the ‘technology node’ for which the lithography step is being used becomes more advanced, that is, as the pattern size and pattern density that need to be generated are becoming respectively smaller and denser such as for example below the 20 nm node. This effect of the difference between mask- and projected-image-pattern is called the ‘optical proximity effect’.
For many years, this effect has been dealt with by applying a so-called ‘optical proximity correction’ (OPC) to the mask pattern: the mask pattern is intentionally made different from the image that one wants to form on the wafer, but in such a way that its projected image becomes closer to the desired wafer pattern. This modification in general implies that the shape of the mask patterns (polygons) is made different from the desired printed shapes in some appropriate way, but it can also imply adding additional polygons to the mask pattern that are not supposed to form printing images on the wafer but somehow improve the process latitudes (see next bullet) of the polygons that are supposed to print. These ‘extra’ mask polygons are often called ‘assist mask features’, or ‘assist features’. OPC has been a standard technology for many years now, and several companies offer software that generates the optical proximity corrected mask pattern, if the intended wafer pattern is given, together with enough details on the way the mask will be exposed in the lithography tool.
The second element that makes optical-projection-lithography more complicated is that the fidelity of the wafer pattern is also affected by the presence of (to a certain extent unavoidable) imperfections in the lithographic process, a number of them being listed here. The printed pattern shape depends on the amount of light that is being used to generated the images on the wafer, the so-called ‘exposure dose’ or ‘dose’. As it is in general not possible to expose exactly at the ideal dose, e.g., due to unavoidable machine- or operator-errors, lithographers aim to work under conditions where they have a sufficient amount of ‘exposure latitude (EL)’, that is conditions under which a certain offset from the ideal dose (usually expressed as a percentage of the dose itself) can be tolerated. The lithographic projection lens forming the image has a so-called ‘best-focus plane’, i.e., a certain plane in space where the image is most ‘sharp’ and hence closest to the intended image. If the wafer is not ideally located with respect to this best-focus plane, one says that the wafer is ‘out-of-focus’. As it is in general not possible to expose wafers exactly in-focus, e.g., due to unavoidable machine- or operator-errors, lithographers aim to work under conditions where they have a sufficient amount of ‘depth-of-focus (DOF)’, that is conditions under which a certain offset from the ideal focus plane (usually expressed by saying how many nanometer the actual wafer plane is away from the ideal plane) can be tolerated. The mask patterns as generated by, e.g., the above mentioned OPC-software can in general not be perfectly realized on the actual photo-mask that is being used in the lithographic process: usually there are ‘mask errors’, i.e., the mask patterns deviate either in size or shape (or both) from the desired mask pattern. Any deviation from the desired mask pattern (‘mask error’) also leads to a deviation of the wafer pattern. As it is in general not possible to avoid mask errors, lithographers aim to work in conditions under which some amount of mask error can be tolerated.
Typically, one needs to find lithographic-process conditions under which enough EL, DOF and tolerance against mask errors is realized. The actual performance of a lithographic process with respect to the mentioned tolerances is often quantified in a metric that is called the ‘critical dimension uniformity’, abbreviated as CDU (expressed in nanometer). This metric expresses how much the dimension of a certain structure in the wafer image will actually vary (e.g., within a printed wafer or from wafer to wafer) due to process variations such as this exposure focus or dose or actual mask errors. Optimization of the lithographic process conditions can then be expressed as a minimization of this CDU metric. An important element therein, although not being the only element, is the selection of the illumination-source shape used in the lithographic exposure tool.
The above mentioned two complications are nowadays often handled by performing a so-called source-mask optimization (usually abbreviated as ‘SMO’). This is a computational process in which the illumination source and the OPCed mask are varied simultaneously in order to find the source-mask combination that offers the best possible or at least sufficient tolerances (also called ‘process margins’) for dose, focus and/or mask error. Several software companies offer automated software to do such an SMO calculation, if the desired wafer image is given, together with other inputs such as the desired process margins and certain parameters that define limitations on the mask complexity that is allowed by the user.
These mask complexity limitations constitute a trade-off between maximizing the lithographic process latitudes and cost. This can be understood as follows. If a calculation is made of what the patterns on the mask should ideally look like to generate an image that is as close as possible to the requested image, the resulting mask patterns are usually extremely complex, which means that the shape of the mask polygons that are intended to generate a printing image as well as the amount, density and shape of the assist-features is very complex. Such complex masks, if at all manufacturable, would be very expensive, as the mask cost increases as the number of polygons that need to be created increases, or if the shape of the individual polygons becomes more complex. Furthermore, using arbitrarily shaped assist features would result in a very large mask file, being the file that describes the exact shape of all the patterns on the mask for which the industry uses a standard file format that is called a gds or gds2 file. Therefore, both OPC- and SMO-software have a number of numerical parameters that influence (limit) the complexity of the solution that the user is willing to accept, and for which the user of the software has to choose numerical value or settings he thinks are appropriate.
The actual complexity of the assist polygons that appear in the final mask solution is further influenced by the values of a set of input parameters for the SMO calculation that are called ‘mask-rule check’ or MRC parameters. The actual values for these parameters (that have to be set by the person who runs the calculation) set limits to the allowed complexity of each individual mask polygon. As an example, these MRC parameters set minimum values to the allowed segment length of mask polygons. An MRC parameter set that allows very complex shapes would be called ‘aggressive’: the printed image shape with such an aggressive set of MRC input parameters would in general be closer to the desired ideal image, but the resulting mask might be non-manufacturable or very expensive. This is why in general people prefer to run SMO software with more ‘moderate MRC’ input parameters, leading to a less perfect printed image but outputting a less complex and hence more manufacturable or less expensive mask.
The decision to allow only rectangular assists limits the mask cost, but it also often leads to smaller process latitudes: many cases are known where the lithographic process tolerances improve if non-rectangular assist-features are allowed as compared to the case where only rectangular assists are allowed. More aggressive input MRC parameters usually also lead to larger process margins.